Lean burn engine systems typically operate at a lean air/fuel ratio significantly lower than the lean misfire limit. This is primarily due to a need to maintain a reserve capacity when controlling fuel injection in response to a load increase. This is especially true for idle speed control for lean burn engines, which is typically accomplished by controlling the fuel quantity/timing and/or the airflow.
One approach for controlling engine idle speed is described in U.S. Pat. No. 6,349,700. In this example, engine/speed control of a direct injection spark ignition engine is accomplished using fuel as a primary torque actuator and airflow as a secondary torque actuator whenever possible to maintain spark near MBT. Fuel is used as the primary torque actuator rather than spark because engine operation is not limited to a narrow range of stoichiometry. When air/fuel ratio limits prohibit the control of torque using fuel, airflow control is used as the torque actuator. Throughout operation, spark is maintained substantially at MBT to enhance fuel economy.
The inventors herein have recognized disadvantages with such a method for engine idle speed control. First, controlling fuel quantity or timing as the primary control for a lean burn engine system results in operation well below the air/fuel ratio lean misfire limit due to the reserve capacity. This reserve capacity can result in decreased fuel economy since operation can occur at a lean air/fuel ratio less lean than otherwise may be possible. Further, engine idle speed control using airflow as the torque control may result in slow engine response.